In an environment in which a host and a storage device are connected (coupled), a technique for controlling an I/O request from the host according to the load of the storage device is developed. For example, Patent Literature 1 discloses returning an error to an I/O request from the host if the system load of the storage device is excessive and causing the host to retry the I/O request. More specifically, in Patent Literature 1, when the load of a specific RAID logical unit (hereinafter, called “RAID group”) is predicted to be high, access to the RAID group is prevented to improve the use efficiency of the RAID group. Therefore, a controller storage device in the system predicts the load of the RAID group based on the I/O request to an externally connected storage device and based on the state of the response to prevent access to the RAID group, in which the load is determined to be high, to improve the use efficiency of the RAID group.